The invention relates to a security system and in particular to a security system including a combination device in which a predetermined combination can be set by an operator to disable the system.
Many such systems are known (see for example U.S. Pat. Nos. 3,953,769, 4,021,796, 4,095,239, 3,633,167, 3,881,171, and 3,878,511) in which each authorised operator is required to remember a predetermined combination or key. To allow for human error, particularly such as might occur under duress during an ambush, it is common to provide for the detection of errors and to permit a limited number of errors prior to disablement of the system.
A more sophisticated solution is for the system to recognise in addition to the usual entry combination, a "duress" code which consists of a predetermined variant of the entry combination when the duress code is entered the system is apparently disabled to permit entry to the secure area but a mute or remote alarm of some kind is actuated to warn of an instrusion. It is an important disadvantage of such systems that the operator is required not only to remember two codes but also to remember the significance of each one. In the interests of operator safety, especially during an ambush, it is important that the intruders do not suspect that a duress code has been used.
In accordance with this invention, we propose a system of this kind in which an alarm circuit is arranged to detect when the set combination differs from the predetermined combination by an error which falls within prescribed limits and, in response to the detection of such an error, to disable the system primary alarm but to generate a secondary alarm, such as a mute or remote alarm.
The system may be a monitoring system including sensors for detecting when a door or window opens or when some other event occurs, so that an alarm is generated when the event occurs unless the correct combination is set; that is to say the system has a passive function.
In a preferred embodiment, however, the system has an active function and acts to perform some operation, such as the unlocking of door or the like.
In either embodiment, the combination is preferably an alpha/numeric word of say ten characters and is set in a keyboard interface unit for comparison by a logic circuit, with one or more stored combinations which are known only to authorised personnel. In this case, a suitable error is 10% this corresponding to a one character error which can be detected by the logic circuit.
Various forms of memory unit have been considered, together with their associated control circuitry. One major consideration was that most memory units require continuous power to retain stored data, and would be reset to zero by a power failure. This risks an intruder being able to gain entry by making the power fail for a short time, but could be avoided by arranging the logic circuit to prohibit the same combination in (any two) memory units. A greater problem associated with memories of this type is reprogramming, which requires complicated control circuits. One memory unit which would avoid power failure problems is a ROM (Read Only Memory) programmed either by fusible internal links, or Ultra-Violet Erasable ROM's, in which the memory is reprogrammed after about 30 minutes exposure to Ultra-Violet. These would require plug-in modules and specialist programming.
Alternative memories include, e.g. Magnetic Tape/disc but these involve excessive power consumption, are mechanically prone to damage, and are bulky, and reprogramming difficulties also exist.
All the above systems could be used with a micro-processor as controller, but these require operating instructions stored, for example, in ROM's. It is preferred, therefore, to use a hard-wired and therefore unreprogrammable control system together with a DIODE MATRIX memory using repeatable numbers to provide a large number of available combinations. This memory, located within the main unit, is programmed by inserting the diode-pins in an array of holes to represent the desired ten character combination.